1. Description of the Prior Art
U.S. Pats., No. 3,646,598, No. 3,817,335 and No. 3,958,647, all in the name of Stephen V. Chelminski, relate to gas discharge type underwater hammers which are improved by the present invention. Each of these hammers includes a vertical tubular sleeve inside of which a massive ram is guided for up and down movement. An anvil is positioned under the ram and is attached to the top of a pile or other element to be driven. A gas discharge device, capable of storing up and suddenly releasing a burst of high pressure gas, is positioned inside the sleeve under the ram. When the gas discharge device is triggered the sudden release of high pressure gas drives the ram upwardly in the sleeve. The ram then falls back onto the anvil and the impact drives the anvil and pile or other element downwardly.
The ram is guided in the tubular sleeve by means of spaced apart sliding shoes mounted on the ram; and the ram circumference itself is smaller than the inside of the sleeve so that an annular clearance exists between the ram and the sleeve. This annular clearance gives rise to problems in that, when the hammer is operated, water will flow down through the annular clearance and onto the anvil while the ram is in flight. Then, when the ram falls back down on the anvil it becomes cushioned by the layer of water resting on the anvil impact surface; and, as a result, a sharp hammer blow is not delivered and the effectiveness of the hammer in driving the pile is less than it would be if the ram were allowed to impact on an anvil surface which was essentially free of water.